Method of fusion sealing wire gird in tube

ABSTRACT

A method is described for mounting a grid within a glass tube wherein the grid is held during assembly in a taut condition, such as by a mounting frame, and is secured between two sections of the tube by glass frit. Also described is an electron tube in which a grid is mounted in such a manner.

United States Patent Inventor Victor G. Johnson La Mesa. Calif.

Appl. No. 19,154

Filed Mar. 13, 1970 Patented Nov. 23, 1971 Assignee Stromberg Datagraphix, Inc. San Diego, Calif.

METHOD OF FUSION SEALING WIRE GIRD 1N TUBE References Cited UNITED STATES PATENTS Cramer et al Badger et 211.. Charlotte Nicklas Mears Primary Examiner-Frank W. Miga Attorney-Anderson, Luedeka, Fitch, Even and Tabin 65/59 X 65/59 X 65/43 X 65/59 X 65/43 X 65/59 X 65/59 X ABSTRACT: A method is described for mounting a grid within a glass tube wherein the grid is held during assembly in a taut condition, such as by a mounting frame, and is secured between two sections of the tube by glass frit. Also described is an electron tube in which a grid is mounted in such a manner.

METHOD OF FUSION SEALING WIRE GIRD IN TUBE This invention relates to electron tubes and, more particularly, to an improved method for mounting a grid in a glass tube, and to an improved tube for construction of which such method is employed.

Various types of electron tubes utilize grids or screens of conductive material in order to regulate or control the flow of electrons in the tube. The grid is connected to a suitable source of potential and is maintained at a convenient potential, which may be fixed or varying.

In constructing an electron tube utilizing a grid, it is important that the grid be securely mounted in the tube and supported where desired in a taut condition. Moreover, since electrical connection must typically be made to the grid from a potential source exteriorly of the glass tube, it is desirable that the electrical connection be adequate to establish a proper potential across the full extent of the grid, and that any leads extending through the wall of the glass tube be hermetically sealed to the glass wall of the tube where they pass through it. The foregoing factors have traditionally presented problems in the manufacture of electron tubes employing grids.

It is an object of the present invention to provide an improved method for mounting a grid in a glass tube.

Another object of the invention is to provide an improved electron tube in which a grid is mounted without the necessity of an internal frame or internal electrical leads.

Another object of the invention is to provide a method for mounting a grid in a glass tube in which the sole support for the grid is the glass tube itself.

It is another object of the invention to provide an improved method for mounting a grid in a glass tube in which the glass tube supports the grid and in which the grid is maintained in a taut condition during the mounting operation.

Another object of the invention is to provide a method of sealing metal to glass where the coefficient of thermal expansion of the metal does not match the coefficient of the glass.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIG. I is an exploded perspective view illustrating a step in performing the method of the invention;

FIG. 2 is a plan view illustrating a subsequent step is performing the method of the invention;

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 2;

FIG. 4 is a plan view of an alternative form of the invention;

FIG. 5 is a sectional view illustrating a subsequent step in the performance of the method of the invention;

FIG. 6 is a sectional view illustrating a still further step in performing the method of the invention and illustrating a portion of an electron tube constructed in accordance with the invention; and

FIG. 7 is a perspective view of a section of an electron tube constructed in accordance with the invention.

Very generally, the method of the invention is for mounting a grid 11 in a glass tube 12. The grid is mounted in a flat condition to extend across the space between two separate sections l4 and 16 of the glass tube 12, placed in axial alignment with each other on opposite sides of the grid. The two sections of the tube are secured together by means of glass solder or frit 17 which fills the spaces in the grid between the facing edges of the tube sections to form a hermetic seal.

Referring now more particularly to FIG. I, the grid 11 is shown disposed in a flat condition for attachment to the two sections of the tube subsequently described. In most cases, the gauge of the wire in the grid does not impart sufficient inherent rigidity to the grid as to maintain the grid in a flat condition. so it is necessary to provide some means for holding the grid in a flat or taut condition for mounting to the tube sections. The particular means employed in the illustrated embodiment is that of a frame 13 consisting of a pair of hoops l8 and 19. The hoops are placed on opposite sides of the grid 11 as shown in FIG. 1 spaced slightly from the grid 11. The hoop 18 is of slightly larger inner diameter than the outer diameter of the hoop 19, thus enabling the hoops to be disposed concentrically. The hoops are smaller than the area defined by the grid in order to provide for the grid's extending completely across the space defined by the frame. 0n the other hand, the space defined or enclosed by the frame is sufficiently large as to exceed the outer dimensions of the tube sections to which the grid is to be attached.

Referring to FIG. 2, the frame 13 is shown in assembled condition for maintaining the grid flat. The procedure is similar to that of the mounting of cloth material on an embroidery hoop. With the hoops l8 and 19 of the frame 13 disposed concentrically, the grid is drawn taut across the space defined by the hoop l9, and is clamped around the periphery of the hoop 19 by the hoop 18. The difference between the outer diameter of the hoop l9 and the inner diameter of the hoop 18 is selected in accordance with the thickness of the grid to provide secure clamping action without damaging the grid.

As an alternative, the frame 13 may be comprised of a single round or square metal hoop. In such a case, the grid 11 is tacked to the metal frame 13 by a suitable series of spot welds. The grid may be gripped with fingers, pliers, or other suitable means and pulled in appropriate directions until sufficiently taut, using a procedure similar to that used by a carpenter tacking a wire screen to a wooden frame door or window. Referring to FIG. 4, this type of frame is illustrated. Parts corresponding to the parts of FIGS. l3 have been given identical reference numerals with the subscript a. The grid 11 is tacked to the frame 13a at the position a by a suitable spot-weld. The grid is then gripped at b and pulled taut in the direction of the arrow, and tacked at c. Additional spot-weld tacks are then placed at the points d so that the entire left side of the frame is attached to the grid. The grid is then gripped at e as before and pulled taut in the direction of the arrow, and then tacked at the position f by a spot-weld. Similarly, it is pulled at the region g and tacked by spotwelding at h, and is pulled at the region i and tacked by spotwelding at the position j. Strengthening spot-weld tacks are added at k. The sides are finished by pulling at points 1 and spot-weld tacking at points m. Final strengthening may be achieved by spot-weld tacking at the points n. In this manner, the grid is held in a flat condition by the frame for the mounting process subsequently described.

A particularly advantageous type of grid which lends itself to the above described spot welding technique on a frame consists of a woven tungsten stocking mesh. This type of mesh is capable of stretching and is thereby not as readily damaged during spot welding. So-called electroformed meshes are fragile and will not stretch. Accordingly, the concentric hoop type of frame support is more successful in connection with these finer meshes. To avoid tearing, the concentric rings should be fabricated with less than 0.00l difference between mating diameters and surfaces should be made very smooth and should be rounded at the edges.

Referring now to FIG. 5, the tube 12 is comprised of two separable sections 14 and 16. The two sections are placed in axial alignment with each other on opposite sides of the grid II, and within the periphery of the frame 13. The two sections 14 and 16 of the tube 12 are secured together by means of glass solder or frit 17 (shown enlarged for purposes of clarity) which fills the spaces in the grid between the facing edges of the tube sections to form a hermetic seal. The grid does not contact the tube sections so that the glass solder is the only sealing agent securing the grid to the tube sections and securing the tube sections to each other.

For securing soft glms, a satisfactory solder glass or frit is Pyroceram No. 89 available from the Coming Glass Co. This material is a devitrifying solder glass (partially crystallizing) and is available commercially as a dry powder. A suspension vehicle of clear amyl acetate with a small amount of nitrocellulose may be used to suspend the powder for uniform application. Brushing, spraying, silk screening and dipping are all satisfactory methods of application. In practice, the unfired solder glass shrinks up to 50 percent or more during the seal ing cycle so that a thoroughly air-dried solder glass layer 1/32 inch is preferably built up on each of the facing surfaces of the sections 14 and 16 to be bonded. Voids within the unfired solder glass contribute to a stressed faulty seal. Accordingly, it is preferable that 1/32 inch layers be built up on the facing surfaces by brushing thin layers of solder glass on each surface and allowing each layer to dry under a heat lamp before the next layer is applied. Ten to 20layers are typically required to achieve the desired thickness. Application is preferably done with the sections disposed such that the facing surfaces are in a horizontal position.

After the aforementioned layers of solder glass are built up on the facing surfaces of the tube sections, the grid ill, in its flat condition, is placed on top of one of the built-up surfaces and solder glass is applied on a layer-by-layer basis to fill up the spaces between the wires of the grid. To do this, a layer of the solder glass in suspension is applied to the screen only in the area immediately over the edge of the glass. This is then dried, and a second layer applied and dried. The process is continued until all the voids in the grid are filled. Two or three layers are typically satisfactory. The last layer is generally preferably of a thin consistency so that it dries more slowly, for the reasons explained below.

After the voids in the grid are filled as above described, the other of the two sections 14 and 16 is inverted and placed on top of the grid in axial alignment with the other section. This is done before the last or thin layer above mentioned is able to dry. If necessary, more glass solder may be applied to fill in the cracks between the aligned tube sections. An excess of glass solder achieves a positive fillet at the joint to avoid the inherent weakness of a crack in the glass surface. After this is done, the unfired seal is air baked for an hour or more to further ensure complete removal of the amyl acetate. Discoloration and a weak porous seal occur if traces of amyl acetate are present during the sealing cycle.

After air baking, the scaling is then begun. The assembly of the two tube sections and the grid is placed in an oven with heating rates consistent with standard soft glass practices and as specified by the-manufacturer of the particular glass solder used. One or 2 psi. may be applied to the seal area to compress the frit or glass solder as it shrinks during the firing cycle. The manufacturer of Pyroceram specifies that the heating rate should not be slower than 2 C. per minute nor faster than 15 C. per minute. In practice, it has been found that the minimum heating rate is only critical during the final approach to sealing temperature. Crystallization can commence before a good seal is obtained if the rate of increase is too low during the final phase of the heating cycle. It is known that a large percentage of typical glass solder is lead oxide and that it is a mixture of different powders. The sealing cycle follows these steps:

I. l00 C.hold for l hour to remove moisture. 2. 2 l0 C.hold for 2 hours for change of lead oxide stage. (Lead oxide changes from less stable oxide to more stable oxide). 3. 325 C.hold for 2 hours for further change of lead oxide to a more stable state and complete decomposition of nitrocellulose. 4. 440 C.hold for l hour to obtain solder glass seal.

The transition from 325 C. to 440 C. should proceed at a rate not less than 2 C. per minute as mentioned above. Sealing takes place at the annealing temperature of soft glass (440 C.). Cool down is at less than l C. per minute until the temperature is below the strain point of the glass. Sufficient oxygen is necessary in the oven to prevent rapid reduction of lead oxide to lead in both the soft glass and the glass solder at the sealing temperature.

Although the invention is not limited thereto, the following criteria provide satisfactory results:

I. A gastight glass solder seal is preferably greater than 0.004 to 0.005 inch thick. Cohesive strength of the molten solder glass is exceeded in a thinner seal and surface tension causes undesirable voids.

2. The diameter of the grid wire is preferably 0.002 inch or less in order to not disturb the dynamic formation of a good glass solder seal.

. For typical glass solder compositions, the holes in the grid should be not smaller than 0.004 to 0.005 inch if a good seal is to be obtained. If finer meshes are to be used, the glass solder powder may be ball milled into smaller initial particles.

4. Where the mesh or grid wires are small compared to the glass geometry, the glas is stronger than the embedded strands and a thermal expansion mismatch between the grid wires and the glass does not typically create problems. Where larger diameter grid wires are to be used, coefficients of expansion should be more closely matched. Further strength is achieved as a result of the devitrified glass solder being stronger than soft glass, and a graded seal results when the grid wires pass through the center of glass solder seal and are equally spaced from the interfaces of the tube sections.

5, The selection of the grid material should not only be satisfactory for the requirements of the electron tube, but should be satisfactory to contend with the particular technique of glass to metal sealing. Important factors are that the grid material have:

a. Low vapor pressure.

b. Low adsorption of gases and moisture.

c. Formation of a metal oxide to act as a bridge between metal and glass.

d. Low oxidation rate at sealing temperatures.

6. Tungsten stocking mesh is generally available with 8m 20 lines per inch and electroformed nickel mesh is generally available with 20 to 2,000 lines per inch.

Although described herein in connection with soft glass, the

invention is applicable to other types of glass envelopes. Glass solders are available for use with a large variety of glasses including ceramics and quartz. The grid material may be the same as above noted, but appropriate selection of glass solder and a corresponding sealing cycle would be necessitated.

Referring now to FIG. 6, subsequently to the glass-soldering operation, the frame 13 is removed. This may be accomplished by any suitable means, depending upon the means utilized to secure the grid to the frame. Where spot-welding is utilized, the grid may be cut by a suitable cutter along a circle around the joined sections of the glass tube. In any case, the result is a useable grid extending over the entire interior cross section of the glass tube or envelope 12.

Since the fragile grid is supported around its entire perimeter after it is fabricated and while the tube is in use, damage to the grid from shocks such as from dropping is readily resisted. Furthermore, the mounting perimeter of the grid is a flat or planar surface, precisely perpendicular to the tube axis. In many prior art tubes, the grid is mounted to a light metal ring or frame, and the frame is then mounted at circumferentially distributed points to metal buttons sealed into the glass tube. If the buttons are not all in one plane, the frame becomes warped and the grid assumes a nonflat condition or becomes nonperpendicular to the axis. This has a deleterious effect on tube performance.

After removal of the frame 113, the portions of the grid extending beyond the outer periphery of the tube 12 may be utilized to provide an external electrical connection to the grid. A convenient way for accomplishing this is to apply an aquadag or conductive cement coating 23 surrounding the tube at the projecting wires of the grid, as shown in FIG. 6 and FIG. 7. The conductive cement coating 23 may be comprised of an epoxy cement filled with finely powdered silver. Electrical connection is then uniform around the periphery of the grid, and suitable external connection may be made by solder leads, clamps, or other appropriate devices, not shown.

Because there is no internal supporting frame or internal leads, the entire interior cross section of the tube is utilized by the grid. The relatively fragile grid is held securely in a taut condition during the mounting process, thereby avoiding damage to the grid. Because of the porous nature of the grid, 21

good strong bond is achievable between the two separate sections of the tube 12. Moreover, because of the free flowing nature of the glass solder during the soldering operation, the spaces between the grid are completely filled to effect the desired hermetic sealing. Thus, manufacture of the electron tube in accordance with the invention provides a tube or superior quality by means of a method which is easily performed at relatively low cost.

While the method of this invention has been described in relation to its preferred use in making a particular product, the method is useful in any application where a metal and a glass of different coefiicients of expansion are to be joined. In many applications, a metal having a matching coefficient is unsuitable because some other property of the metal is undesirable. For example, when a metal electrode is to be attached to the inside of an evacuated glass envelope, metals having high vapor pressures, high secondary emission ratios, or magnetic properties are often undesirable. A metal having a mismatched coefficient of expansion, which is otherwise suitable, may be successfully joined to the glass by dividing or forming the portion of the metal piece to be attached into a number of spaced apart fine strandsv The expansions and contractions of the fine strands, each embedded in much larger section of glass frit, are incapable of fracturing either the glass frit or the glass of the envelope. The greater the difference in the coefficients of expansion, the smaller the strands which are required for a successful joint.

It may therefore be seen that the invention provides an improved method for mounting a grid in a glass tube, and further provides an improved electron tube during the construction of which such method is employed. The grid is supported solely by the tube, having no internal support or internal electrical leads. Hermetic sealing of the tube is readily effected, and good electrical contact with the grid may be achieved without additional leads.

Various modifications of the invention, in addition to those shown and described herein, will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

l. A method for mounting a grid of closely spaced fine wires across a glass tube which comprises the steps of:

a. supporting said wire grid in a flat condition;

b. coating the mating surfaces of two sections of glass tubing with a solder glass suspension;

c. bringing said flat grid into contact with the solder glass layer on one of said surfaces;

d. coating said grid with said solder glass suspension in those areas immediately over the glass tube surface until interwire spaces are substantially filled with said solder glass;

e. bringing the mating surfaces of the two tube sections together with said grid therebetween; and

f. heating said solder glass until a hermetic seal is formed between said tube sections and said grid.

2. The method according to claim 1 including the further steps of removing any external gn'd support means and applying a continuous conductive coating to the portions of said grid which project exteriorly of said tube.

3. The method according to claim 1 wherein said coating steps are perfonned by dispersing a finely divided solder glass in a volatile organic carrier and brushing on a plurality of layers of said suspension with at least partial air-drying between layers, the number of layers applied being sufficient to produce a seal having a thickness of at least about 0.004 inch.

4. A method according to claim 1 wherein the grid is placed in a flat condition by securing it in a taut condition to a mounting frame which is larger than the cross section of the tube so that the grid extends across the space defined by the frame, and wherein the sections of the tube are aligned with each other within the periphery of the frame.

5; A method according to claim 4 wherein the grid is attached to the frame by spot weldin g. It 

2. The method according to claim 1 including the further steps of removing any external grid support means and applying a continuous conductive coating to the portions of said grid which project exteriorly of said tube.
 3. The method according to claim 1 wherein said coating steps are performed by dispersing a finely divided solder glass in a volatile organic carrier and brushing on a plurality of layers of said suspension with at least partial air-drying between layers, the number of layers applied being sufficient to produce a seal having a thickness of at least about 0.004 inch.
 4. A method according to claim 1 wherein the grid is placed in a flat condition by securing it in a taut condition to a mounting frame which is larger than the cross section of the tube so that the grid extends across the space defined by the frame, and wherein the sections of the tube are aligned with each other within the periphery of the frame.
 5. A method according to claim 4 wherein the grid is attached to the frame by spotwelding. 